Fluidized bed coking (fluid coking), including its variant, the Flexicoking™ process, is a pyrolysis process used in the petroleum refining industry in which heavy petroleum fractions, typically the non-distillable residue (resid) from vacuum fractionation, are converted to lighter, more useful products by pyrolysis (coking) at elevated reaction temperatures, typically about 500 to 600° C. (approximately 900 to 1100° F.). In fluid coking, the heated heavy oil feed, mixed with atomizing steam, is admitted through a number of feed nozzles to a large vessel containing coke particles fluidized with steam and maintained at a temperature high enough to carry out the desired cracking reactions in the reactor section of the vessel. The feed components not immediately vaporized coat the coke particles and are subsequently decomposed into layers of solid coke and lighter products which evolve as gas or vaporized liquids which mix with the fluidizing steam and pass upwardly through the dense fluidized bed of coke particles, through a phase transition zone into a dilute phase zone above. The solid coke consists mainly of carbon with lesser amounts of hydrogen, sulfur, nitrogen, and traces of vanadium, nickel, iron, and other elements derived from the feed material. The fluidized coke is continuously withdrawn from the reactor vessel, steam-stripped and circulated through a burner, where part of the coke is burned with air to raise its temperature from about 500 to about 700° C. (about 900 to 1300° F.), after which it is returned to the reactor vessel to provide heat for the coking reaction.
The mixture of vaporized hydrocarbon products and steam continues to flow upwardly through the dilute phase at superficial velocities of about 1 to 2 metres per second (about 3 to 6 feet per second), entraining some fine solid coke particles. The gases then pass upwards out of the reactor section of the vessel through separator cyclones into a scrubber section. Most of the entrained solids are separated from the gas phase by centrifugal force in the cyclones and are returned through the cyclone diplegs to the dense fluidized bed by gravity. The mixture of steam and hydrocarbon vapor is discharged from the cyclone outlet and quenched to about 370-400° C. (about 700-750° F.) by contact with circulating oil in the scrubber section of the fluid coker vessel. The scrubber is equipped with internal sheds normally in the form of inverted U- or V-shaped elements, to facilitate contact between the ascending vapors and the oil passing down from a distributor above the sheds. The contact between the high boiling circulating oil and the ascending vapors provides cooling of the hot vapors and promotes condensation of the heaviest fraction of the vaporized product. A de-entrainment section is also conventionally provided above the sheds with additional wash oil provided from a distributor located above the de-entrainment device. The de-entrainment device acts to remove entrained heavy oil droplets from the vapors and to cool the vapors further; it is important to the quality of the final coker gas oil product that the de-entrainment device should not accumulate coke particles and other impurities which can be entrained by the passing vapors. Heavy oil and solids and liquids separated in the scrubber section pass out at the bottom of the scrubber section to a pumparound loop which circulates condensed liquid to an external cooler and back to the top of the sheds in the scrubber section. This heavy fraction is typically recycled to extinction by feeding back to the fluidized bed reaction zone, but may be present for several hours in the pool at the bottom of the scrubber section.
Fluid coking is an established process and is described briefly, for example, in Modern Petroleum Technology, Hobson, G. D. (Ed.), 4th Edition, Applied Science Publ. Ltd., Barking, 1973, ISBN 085334 487 6.
The gas phase undergoes a pressure drop and cooling as it passes through the cyclones, primarily at the inlet and outlet passages where gas velocity increases. The cooling which accompanies the pressure decrease causes condensation of some liquid which deposits on surfaces of the cyclone inlet and outlet. Because the temperature of the liquid so condensed and deposited is higher than about 500° C. (about 900° F.), coking reactions occur there, leaving solid deposits of coke. Coke deposits also form on the scrubber sheds, the de-entrainment device and other surfaces. In particular, fouling of the de-entrainment device, normally a grid, restricts the open flow paths in the grid and eventually leads to flooding and black oil entrainment. A poorly operating scrubber can readily lead to poor product quality since this is determined in part by scrubber operation: heavy ends which contain metals, Conradson Carbon Residue (CCR) and, in the case of tar sand operations, fine clay solids, can enter the coker products, leading to problems in downstream units, particularly catalytic units such as hydrotreaters in which metals such as vanadium and nickel can poison the catalyst and entrained clay solids plug catalyst beds and cause high pressure drop.
One pathway by which fouling of the scrubber sheds and of the de-entrainment device is believed to arise is coking of heavy oil entrained in the scrubber section by the high velocity gas flow from the cyclone outlets. The heavy components in the oil carried up from the sheds impact the de-entrainment device and then become coked as a result of high temperatures prevailing in the scrubber. At the end of a run, this fouling can be so bad that the de-entrainment device loses its effectiveness as a contact device: it floods, and allows heavy components from the circulating oil into the product stream. This problem, moreover, becomes more severe as the degree of fouling increases and the gas flow passages become progressively smaller, the gas flow in the de-entrainment device then becomes correspondingly faster and entrainment into the product from the unit sent to downstream units, in turn, increases yet further.